Proximity switches of the aforementioned type are generally provided with an HF (high frequency) resonance circuit which is adapted to be influenced by a magnetic trigger, in an oscillator circuit, with a coil arrangement having a member magnetizable by the magnetic field of the trigger. The member is drivable into magnetic saturation starting with a specific magnetic field strength by the external magnetic field of the trigger with reduction in the damping of the resonance circuit. The magnetizable member may, for example, consist of an amorphous or predominantly amorphous metallic band of high permeability, with losses of the band, which determine the oscillation state of the oscillator circuit, decreasing with an increase in magnetic saturation.
Various embodiments of a proximity switch of the aforementioned type are proposed in, for example, EP Laid-Open Application 0,218,042 wherein, in all of the embodiments, the coil arrangement includes a resonance circuit coil and a coupling coil arranged on a joint bobbin. In one of the proposed embodiments a "Hartley Circuit" is utilized for the oscillator, and in the other proposed embodiment, a Meissner circuit is employed.
One common feature of the above mentioned conventional circuits used in such proximity switches resides in the fact that the circuit oscillators operate with two windings or with a tapped coil since these types of circuits, as is known, can be dimensioned with special ease so that a uniform reproducible starting and stopping of the oscillation is readily obtainable, with a corresponding amplitude curve being respectively produced during the damping reduction and damping of the coil arrangement.
However, a disadvantage of the above noted conventional circuits resides in the fact that two windings or coil taps are required during the manufacturing which not only increases the manufacturing costs but also a further processing of the coil with, for example, three or four terminals, requires a considerable amount of operational steps and labor and is also prone to errors. Moreover, the basic losses in the conventional coil arrangement are high and, for this purpose, the use of conventional coil arrangements presents some difficulties especially in integrated switching circuits.